What this calculator works out
Fencing a garden comes down to three numbers most people want before they order anything: how many panels or sections to buy, how many posts to set, and how much Postcrete, Quikrete or loose concrete each post needs. This tool takes the total run, the panel width, the fence height, and how many gates break the run up, and turns all three of those into a straight shopping list, along with an optional gravel board count and an itemised cost if you know your local prices.
It works for a UK garden using 6ft (1.83m) panels and Postcrete, an American yard using 8ft sections and Quikrete Fast-Setting Concrete Mix, or anything in between: every measurement has its own metric or imperial toggle, so a run measured in metres and a panel bought in feet can sit side by side without any conversion by hand.
How the maths works
Gates come out of the run first, because a gate opening does not get a panel. Whatever is left is divided by the panel width and rounded up to the next whole panel, since no supplier sells part of one.
Posts come from the number of bays, not the number of panels on their own. A "bay" is any gap between two posts, whether it holds a panel or a gate, so the bay count is panels plus gates, and a straight run always has one more post than it has bays, the extra post closing off the far end.
That last rule is the one people usually get wrong, and it is worth stating plainly: a gate does not need extra posts beyond the two that already bound its bay. Those two posts just have to be sturdier, because they carry the gate's weight and the swinging force every time it opens, which is a build-quality decision rather than a counting one.
Bags per post are then a straight multiplication, rounded up once at the end across every post rather than post by post, since a part-used bag on one hole is still useful for the next.
A worked example
Take an 18m run of UK 6ft panels (1.83m each), one 1m gate, a 6ft fence height, and 100mm posts set in Postcrete. The gate takes 1m off the run, leaving 17m of panel run; 17 ÷ 1.83 = 9.29, rounded up to 10 panels. That is 10 panels plus 1 gate, 11 bays, so 12 posts. At 0.75 bags of Postcrete per 100mm post (Tarmac's own datasheet figure for a 20cm by 60cm hole), that is 12 × 0.75 = 9 bags. At a rough £45 a panel, £14 a post and £5.60 a bag, materials come to (10 × 45) + (12 × 14) + (9 × 5.60) = 450 + 168 + 50.40 = £668.40, before labour.
The American equivalent: a 60ft run of 8ft sections (2.44m), one 3ft gate, a 6ft fence, and 4in posts set in Quikrete Fast-Setting Concrete. The gate takes 3ft off, leaving 57ft; 57 ÷ 8 = 7.125, rounded up to 8 sections, 9 bays, 10 posts. At 3.5 bags per 4in post, that is 35 bags of Quikrete. At $70 a section, $18 a post and $9 a bag, materials come to (8 × 70) + (10 × 18) + (35 × 9) = 560 + 180 + 315 = $1,055, again before labour.
Choosing a post-setting method and depth
Tarmac's Postcrete datasheet publishes an exact table of bags against hole size and post section; a 100mm square post in a 20cm (8in) wide, 60cm (24in) deep hole needs 3⁄4 of a 20kg bag, and a 75mm post in the same width hole needs a full bag at 60cm deep or 3⁄4 at 45cm. Quikrete's Fast-Setting Concrete Mix datasheet gives a different kind of figure: each 50lb bag yields about 0.375 cubic feet, the hole diameter should run roughly three times the post's width, and the hole depth should be about a third of the post's height above ground, with two 50lb bags stated as typical for a 3in post in a 9in wide, 30in deep hole. Both figures are baked into the defaults here, keyed to the post size you pick; the bag count itself stays an editable field, because soil, hole shape and how tightly a hole is packed all move the real number a little.
The wider rule both manufacturers point to is the same: dig to roughly a third of the post's height, and never shallower than about 450mm (18in), which is the commonly cited floor for a stable domestic fence post regardless of the exact height above ground. A run in an exposed, windy spot, at the top of a slope or facing open fields, is worth digging deeper and using the larger post section throughout, not just at the ends, since wind load on a solid panel fence is considerable and a shallow-set post is the most common reason a fence goes over in a storm.
The third method, dig and concrete by volume, skips bags altogether and works out the net concrete volume needed per hole, hole volume minus the post's own volume, using the same three-times-post-width and height-based depth guidance, for anyone buying ready-mix, a bulk bag, or a different bagged product entirely.
Panels, posts and gates: getting the count right
Post spacing is not a separate figure to work out; it is simply the panel width, since each post sits at the join between two panels (or a panel and a gate). Choosing a narrower panel width automatically means more, closer-spaced posts for the same run, and a wider one means fewer, further apart. The one thing this calculator assumes is a single straight run with a post at each end; an enclosed rectangular garden run has no free ends, so its post count equals its bay count exactly, one fewer post than the straight-run formula above gives, because the last post is shared with the first.
Gravel boards
A gravel board is a short board, usually timber or concrete, that sits along the bottom of a panel bay and lifts the panel itself clear of the soil, commonly by around 150mm (6in). Keeping the panel off wet ground slows the rot that starts at a fence's lowest edge, and it means only the board, not the whole panel, needs replacing if the bottom does eventually go. One board per panel bay is the standard count; a gate opening does not carry one, since there is no panel there to protect.
Questions people ask
How many fence posts do I need for a given run?
Work out the number of bays first, panels plus gates, then add one for a straight run with two open ends. A 30m run of 1.83m panels with one gate comes to 16 panels, 17 bays, and 18 posts.
Do gates need extra posts?
No. A gate sits in its own bay between two ordinary boundary posts, the same as a panel would. Those two posts usually need to be sturdier to carry the gate's weight and swinging force, but the count does not go up.
How deep should a fence post hole be?
Roughly a third of the post's height above ground, and rarely shallower than about 450mm (18in) even for a short fence. Go deeper, and consider a larger post section, on exposed or windy sites.
How many bags of Postcrete does one fence post need?
Tarmac's own datasheet works this out by post size and hole size rather than a single fixed number: a 100mm post in a 20cm by 60cm hole needs about 3⁄4 of a 20kg bag, and a 75mm post needs a full bag at the same width and depth. This calculator defaults to those figures and lets you edit the number directly.
How many bags of Quikrete does one fence post need?
Quikrete's Fast-Setting Concrete Mix datasheet states two 50lb bags for a 3in post in a 9in wide, 30in deep hole, with each bag yielding about 0.375 cubic feet. A larger 4in post in a wider hole needs proportionally more, which is where this calculator's default for the larger post size comes from.
Do I need gravel boards?
They are optional but common good practice: a gravel board keeps a panel's bottom edge off wet soil and is cheap to replace on its own if it eventually rots, rather than the whole panel needing to come out.
This calculator uses published sizing rules from Tarmac's Postcrete datasheet and Quikrete's Fast-Setting Concrete Mix datasheet, alongside standard UK and US panel dimensions. Real bag counts vary a little with soil type, hole shape and how firmly a hole is packed, so treat the figures here as a solid planning estimate, round up when buying, and check your own supplier's guidance for a large order.